Equine infectious anaemia
The course of the disease depends on the general condition of the animal: Fever over 40 °C, loss of condition and weight, swaying on the hindquarters, often oedema of the limbs, abdomen and lower chest. Equine infectious anaemia may present as peracute (sudden death), acute or chronic manifestations. The acute form of the disease is usually fatal, and animals may die within a few days. In the chronic form, the disease can last for several years.
Equine infectious anemia (EIA) was first diagnosed in France in 1843. The causative agent of equine infectious anemia, Equine infectious anemia virus (EIAV), is a lentivirus of the Retroviridae family, subfamily Orthoretrovirinae. Hosts of EIA are all odd-toed ungulates of the equine family (Equidae), i.e. horses, ponies, mules, donkeys, zebras, etc.
The virus is not considered a risk to human health, so according to current knowledge there is no risk of infection to humans.
The EIA virus is distributed worldwide and occurs in North, Central and South America, South Africa, Northern Australia, Japan, Great Britain, Belgium, Germany, France, Italy, Romania, Slovenia, Croatia, Hungary and Greece. The outbreaks reported in the first half of 2010 in Belgium, Germany, France and England could be attributed mostly to horses brought from Romania (SCoFCAH 2010). In Romania, the disease is considered endemic and therefore strict rules apply to the transport of live horses from Romania. The EIA virus can be inactivated by commercial disinfectants. Suitable cleaning or disinfecting agents include fat solvents, strongly acidic and alkaline compounds; however, aldehydes such as formaldehyde and glutaraldehyde are not suitable.
Infectious equine anaemia is mainly transmitted by blood-sucking insects such as mosquitoes, biting flies (maggots) and horseflies. The EIA virus survives on the mouth parts for only 30 minutes to a few hours and is not replicated in the insect. Transmission over longer distances (more than 200 m) has not yet been observed. Transmission rates increase in late spring, culminate in late summer and early fall, and are lowest in winter. In river valleys, swamp and woodland areas, the incidence of disease is highest after grazing, and lower on high pastures and where cattle are kept indoors. Regional and seasonal distribution patterns are explained by insect occurrence and population density. Transmission from horse to horse requires close animal contact. The virus reservoir is formed by chronically or symptomlessly ill horses, which are lifelong carriers of the virus. These animals are potential sources of infection for virus introduction into disease-free herds. Transmission can occur during the reproductive act, in pregnant mares via the placenta to the foetus, via breast milk and via excretion of saliva and urine. Transmission is also possible through nursing utensils contaminated with blood and secretions or through the muzzle, as well as via injection cannulae, surgical instruments and unlicensed blood and plasma products.
The course of the disease depends on the general condition of the animal. The incubation period is 1-3 weeks, rarely longer than 3 months.
Equine infectious anaemia may present as peracute (sudden death), acute or chronic manifestations but may also be largely asymptomatic. The acute form of the disease is usually fatal within a few days. In the chronic form, the disease can last for several years.
The acute form of the disease manifests itself in fever, apathy, weakness, disturbances in the coordination of movement (= ataxia), jaundice (= icterus), palpitations, cardiac arrhythmia and bleeding from the capillaries of the mucous membranes (= petechiae), which occur mainly on the underside of the tongue and on the conjunctiva of the eyelids. The internal body temperature rises to over 40°C. The fever may last for one day, but may also last for several days. The animals are dull, become anaemic and show congested, sometimes icteric mucous membranes as a result of the onset of cardiac insufficiency. Loss of condition and weight are the result. Diseased animals may sway on the hindquarters and often show edema of the limbs, abdomen, and lower chest. Pregnant mares may have abortions and give birth to weak foals. In case of therapy-resistant fever episodes as well as thrombocytopenia (= reduced number of thrombocytes), EIA should always be clarified as a differential diagnosis.
The chronic form is characterized by episodes of illness with fever attacks, exhaustion and edema on the underside of the abdomen. The disease episodes, which occur at intervals of 4-6 weeks, last 2-5 days. After that, the animals are symptom-free again until a new episode of the disease occurs. With increasing duration of the disease, the animals develop anemia and hypergammaglobulinemia (= increased content of gamma globulins in the blood). They eventually become so weak that they can no longer stand. The animals become emaciated despite feed intake.
An infected animal must be considered a lifelong virus carrier (persistent viremia) and potential virus excretor, regardless of whether a clinical manifestation is observed. 30 to 90 % of infections often proceed for a very long time without apparent symptoms of disease. The anemia that gives the disease its name is often not observed - it results from immunopathological breakdown of red blood cells.
To date, there is neither a treatment nor an effective vaccine. EIA is notifiable in all EU member states and is one of the notifiable animal diseases to the World Organisation for Animal Health (OIE). When purchasing horses from anaemia regions, an antibody test should be carried out beforehand to exclude EIA.
Suitable sample material for antibody tests is blood (serum).
The diagnosis of EIA is based on serological detection methods of antibodies against the EIA virus (Coggins test or ELISA method), as the PCR method currently neither approaches the sensitivity and specificity of serological detection methods nor represents a routine method in the diagnosis of this disease. Antibodies are detectable by Coggins test at the earliest 2 to 3 weeks, rarely more than 60 days after infection.
Positive cases can be confirmed by PCR (polymerase chain reaction). The conserved gag gene is used as a target. However, neither the OIE nor the EU reference laboratory ANSES recommends a specific PCR method for the detection of EIA due to the high genetic variability.
Serum, EDTA-blood as well as organ samples are suitable as sample material for the PCR examination. Differential diagnoses include equine viral arteritis, African horse sickness, piroplasmosis, babesiosis, ehrlichiosis, leptospirosis, petechial fever, kidney or heart disease, chronic bacterial infections, severe parasite infestations or gastrointestinal catarrh.